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/ NLCTA-Note #48
May 12, 1995
Subject: Average Current Limit for the NLCTA Thermionic Gun.
Authors: M. Brown

This is a brief description of the current limiting circuitry for the NLCTA Thermionic Gun Pulser. This circuitry used in addition to Repetition Rate Limiting accomplished with a retriggerable monostable. We will start with a description of the pulser circuit. Then we will provide plots of the expected performance.

PULSER CIRCUIT

A simplified schematic of the Gun Pulser circuit is shown in figure 1. Working back from the gun cathode we see a planar triode grounded grid amplifier. This triode provides two main functions. First, this tube is run "pervience limited" to improve the output rise and fall time. Second and probably more important, this tube isolates damaging gun arcs from the semiconductor circuitry. The current gain of the planar triode amplifier is ²1. Therefore, the available current is determined by the FET Switch circuit.

Pulse Current: The series, 90½ resistor that drives the cathode of the output triode determines the pulse current. Allowing for tube bias, FET "on" resistance and cathode input impedance a value of 90ʽ provides approximately 3 Amps of output current. This value can be changed if a higher pulse current is desired.

Average Current: The FET Switch is fed from an active shunt regulator circuit. The operational amplifier drives a regulator FET. The shunt regulator is fed through a 112Meg series resistor. This resistor limits the current available to the FET Switch. The 112Meg resistor is made up of 2 sections to increase the creepage path, i.e., the distance over which leakage current might flow. Furthermore, these resistors have grounded guard traces around them. These ground traces force leakage currents to flow to ground rather than around the resistors. Any leakage thus reduces the available output current. No component failure other than the 112 Meg and 199 Meg resistors can increase the available current. Failure of other components reduce or eliminate the output current. The effect of the 199 Meg is minimal. If this resistor opens, the available output current will increase by 1.7 µAmps. The 1000 volt Supply is an unregulated DC to DC converter whose output is proportional to its input. The source for this supply is the 12 Volt NIM supply on the HV deck. All Deck NIM voltages are monitored by the control system.

CIRCUIT SIMULATION

Figure 2 shows a simplified drawing of the circuit. This circuit was simulated using PSpice. In the simulation the 199 Meg resistor was given a value of infinity. The gain of the output stage was assumed to be unity and that all of the current from the FET Switch reaches the gun cathode. Figure 3 shows a plot of the supply voltage as a function of time at a 60Hz repetition rate. Figure 4 and 5 show the dependence of average current and pulse current on rep-rate. The maximum average current is limited to approximately 11 µAmps at 180 Hz. If we take into account the effects of the 199 MEG resistor and the planar triode bias the actual available output current will be about 2 µAmps lower. This is about the lowest value of supply current we can use for stable 10Hz operation given the fact that the gain of the output stage will probably be less than unity and bias offset and leakage current will lower the output further. The gain of the output stage will depend on the anode voltage we end up using. This voltage will be adjusted to optimize pulse shape. (rise time, fall time and droop)

OPERATIONAL TESTING

During normal operation the repetition rate will be limited to 10Hz by the retriggerable monostable in the Pulse Trigger Driver Circuit. To test the effectiveness of the average current limiting circuitry, we can increase the trigger pulse width to 1.2 µseconds. This creates an average current approximately the same as a 200 nanosecond pulse at a 60 Hz rate. This should cause a decrease in the pulse current by more than a factor of 3. This change in pulse current will take about 3 seconds to occur. Figure 6 shows the time response of an instantaneous increase in pulse width from 0.2µSeconds to 1.2µSeconds.

Fig. 3 Output Voltage as a Function of Time at 60Hz

Fig. 6 Output Voltage and Current as a Function of Time with Wide Pulse (1.2µSec)